Method of treating magnesium metal article prior to spotwelding



United States Patent 3,087 841 METHOD OF TREATING MAGNESIUh I METAL ARTICLE PRIOR TO SPOTWELDING Herbert K. De Long, Midland, Micl1., assignor to The Dow Chemical Company, Midland, Mich, a corpora- (ion of Delaware No Drawing. Filed Nov. 3, 1958, Ser. No; 771,206

3 Claims. (Cl. 13427) The invention relates to an improved method of preparing the surfaces of an article of magnesium and its alloys prior to spotwelding.

Magnesium and magnesium-base alloys comprising at least 85 percent of magnesium for the purposes of the specification and claims are referred to hereinafter as a magnesium metal.

In preparing the surface of a magnesium metal article for electrical resistance spotwelding it is essential to remove the normal oxide layer, and especially mill scale formed as in hot mechanical working, in order to provide a. surface of low resistivity. Attempts to spotweld without removing such surface contaminants result in intense local heating and burning of the magnesium metal article at the weld site as well as rapid oxidation of the welding electrodes because of the resistance of the contaminant layer to current flow.

Mechanical methods for cleaning magnesium metal articles have been employed in the past, such as, wire brushing, or abrading with steel wool. Chemical cleaning or pickling methods have also been used, for example, pickling magnesium metal articles for several minutes in an aqueous solution containing about 18 percent of chromic acid and 0.05 percent of sulfuric acid as described on p. 36 of the handbook Magnesium Finishing, published in 1955 by The Dow Chemical Company. While the cleaning methods heretofore proposed have been generally effective in cleaning magnesium metal articles and producing thereon a bright metal surface evident upon visual inspection, none of the said methods has been entirely satisfactory in that after a relatively few welds have been made, such as up to 50 or 60 welds, but frequently less than 30 welds using the same electrodes they tend to become stuck to the magnesium metal articles being joined so that rough weldspots are obtained and the electrode tips become pitted. If more welds are made with the pitted electrodes progressively poorer Welds are obtained and the electrodes deteriorate further as by oxidizing, sticking and pitting until burning, or flashing, occurs during the welding cycle producing a visually observable hole or pit through the magnesium metal surface. In addition, when the welding electrodes, which are normally formed of copper, stick to a magnesium metal article some of the copper from the electrodes adheres to the article. The corrosion resistance of the so-contarninated magnesium metal article is thereby seriously impaired since the galvanic cell action of a copper-magnesium metal couple in a corrosive medium is very pronounced. Copper contamination of magnesium metal articles is to be avoided. It is therefore practically mandatory to clean and reshape the electrode tips upon observing the occurrence of sticking or pitting of the electrodes. Since it is necessary to stop the welding operation to clean and smooth the electrodes, down time of the welding equipment has been a problem. It is thus apparent that ordinary cleaning methods alone are not adequate.

It has now been discovered that by subjecting a magnesium metal article having a clean and bright metal surface to the action of a dilute aqueous solution of chromic acid the welding electrodes do not become fouled by the so-treated magnesium metal surface but may be used many times without having to be cleaned or reshaped and still obtain acceptable welds. The invention then 'ice consists of the improved method of treating a magnesium metal article prior to spotwelding hereinafter more fully described and particularly pointed out in the claims.

In carrying out the invention a magnesium metal article having a clean and bright surface is subjected to the action of a dilute aqueous chromic acid solution, as by immersion, for an interval of five seconds to three minutes, a generally effective immersion time being from 15 to 45 seconds. The proportions of chromic acid (CrO in the aqueous chromic acid solution employed are from about 0.5 to 10 grams per liter of solution, though concentrations from about 1.5 to 3 grams are to be preferred. A solution temperature in the range of about 60 to 212 degrees Fahrenheit is suitable for the treatment, a temperature in the range of about 70 to 100 degrees Fahrenheit being preferred. The so-treated article is then rinsed with water and dried as by air drying. Quick drying is facilitated, in practice by rinsing the treated magnesium metal article with hot water following a cold water rinse.

During handling and storage a clean and bright magnesium metal article rather quickly builds up a surface oxide film which is not readily penetrated or removed by the dilute chromic acid solution of the invention. Therefore while the invention may be utilized in treating a magnesium metal article having a clean and bright metal surface it is to be preferred that a magnesium metal article he freshly cleaned, as by conventional methods, before treating it according to the invention.

The cleaning step may be carried out in any one of several different ways to attain the objective of making the surfaces of the article clean and bright. For example the article to be spot welded may be mechanically cleaned as by wire brushing, chemically cleaned as by subjecting the article to the action of an acidic aqueous cleaning composition, or, if relatively free of mill scale and the like, it may simply be degreased by the action of an organic solvent such as a halogenated hydrocarbon, of which perchloroethylene is an example. Of these cleanmethods, the use of an aqueous acidic chemical cleaning composition is to be preferred as producing the most rapid and positive cleaning action in addition to producing generally superior results measured in terms of the number of spotwelds which may be made with the same electrodes on so-cleancd articles following treatment in the dilute chromic acid solution in accordance with the invention.

Example of suitable aqueous acidic chemical cleaners for use in obtaining a clean bright surface are: (l) a water solution containing about 0.5 to 1 percent of H (2) a water solution containing about 18 percent of CrO plus 0.05 percent of H 50 and, (3) a water solution containing a water-soluble organic carboxylic acid and a water-soluble nitrate salt selected from an alkali metal nitrate, an alkaline earth metal nitrate, or ammonium nitrate.

The carboxylic acids most suitable for use in the third chemical cleaner composition above cited are oxalic acid, aliphatic carboxylic acids such as formic, acetic, and propionic acids, and hydroxy aliphatic carboxylic acids such as glycolic, lactic, citric and tartaric acids. Acetic, glycolic, citric and tartaric acids are particularly effective. The proportion of carboxylic acid used may range from about 5 to 45 percent by weight though proportions in the range of about 15 to 25 percent are preferred except in instances where the surface to be cleaned is heavily contaminated with graphite in which case acid concentrations of about 30 to 40 percent are preferred. The watersoluble nitrate salt to be used is added to the carboxylic acid solution in an amount affording a nitrate (N0 concentration of about 1.5 to 12 percent by weight. A generally useful N0 concentration is about 2 to 6 percent of cleaner composition.

In carrying out the cleaning step using an aqueous acidic chemical cleaner, as exemplified above, the mag-- nesium metal article may first be degreased, if desired, as by subjecting it to the action of an aqueous alkaline solution followed by water rinsing to remove excessive oil. or grease contamination as described on p. 31 of the hand-- book Magnesium Finishing referred to hereinabove. The article is then immersed or otherwise subjected to the action of one of the aqueous acidic chemical cleaners of the types described above for a time and at a temperature sufiicient to remove the surface contaminants but insutfi cient to permit any substantial attack upon the underlying metal. The duration of the cleaning step is not critical and may be easily gauged by visual examination of the magnesium metal article during the operation. The metal surface usually becomes clean and bright within about 1. minute upon carrying out the operation at room tempera ture, though higher temperatures may be used. The socleaned magnesium metal article which then exhibits a bright surface is removed from the aqueous acid chemical cleaner, rinsed with water, and drained at least momentarily before treating it in accordance with the invention.

The mode of action of the dilute chromic acid solution is not entirely understood though it appears that it not only exerts a mild cleaning action on the magnesium metal article but it also deposits a very light protective chromium-containing film on the magnesium metal surface. The protective film is believed to maintain the magnesium metal surface in a condition that is adapted to be welded, and, in addition, the film reacts with the welding electrodes during the welding cycle removing oxidation products of copper therefrom. Thus the re-- sistivity of the electrodes is prevented from building up rapidly, better spot welds are obtained, and sticking of the electrodes to the magnesium article is less likely to occur.

A series of tests were conducted for the purpose of comparing various embodiments of the invention with cleaning and treating methods heretofore proposed. The various methods were evaluated on the basis of their effects on subsequent spotwelding of so-cleancd articles. In each test there were employed one or more pairs of pieces of alloy sheet each having the dimensions 12 inches x 12 inches x 0.064 inch and formed of a magnesium-base alloy having the ASTM designation AZ31, the alloy sheet having previously received a conventional pickling treatment, according to U.S. Patent 2,607,739, when rolled into sheet form. In carrying out the test series, pairs of pieces of alloy sheet were cleaned according to various methods as listed in the table and each pair, respectively,

was thereafter aligned in intimate face to face relationship and spotwelded together as described below.

A spot weld machine equipped with conventional 1 inch diameter rod-shaped copper electrodes having convex, or dome-shaped tips, the radius of curvature of the convexity being about 4 inches, was operated as follows:

Electrode force 1800 lb.

Pulse 3.

Cycles 3.

Current 40 percent.

Polarity Normal (full cycle).

Under these conditions spotwelds of about 0.3 inch diameter were produced. The spot welds were spaced in rows on 1 inch centers, successive rows being /2 inch apart and the centers staggered at about the maximum distance from the spots in the preceding row. Starting with clean, freshly-shaped electrodes, a series of spotwelds was made and the series continued until burning or flashing occured to produce a damaging hole or pit through the metal surface as observed visually, or, in some of the tests, until the electrode tips stuck to the alloy sheet three consecutive times. In some tests in which many good welds were made the series of spotwelds was continued on other identically treated pairs of pieces without stopping to clean the electrodes. The cleaning and treating methods are summarized in tabular form in the table along with the number of good welds made on each pair of alloy pieces so-cleaned. In tests 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, ll, 12 and 13, the alloy pieces after cleaning received a post treatment in chromic acid solution according to the invention. In tests 14, 1S, and 16, post treatment of the alloy pieces in chromic acid solution was omitted. The test results show that post treatment, in accordance with the invention, of conventionally or otherwise treated magnesium metal articles with a dilute aqueous solution of CrO prior to spotwelding is beneficial in that a greater number of satisfactory spotwelds can be made without having to clean and reshape the electrodes than when the post treatment is not used. The test results, e.g., 1, 4, 8, 9, 10, and 13, also show that an especially effective combination of procedural steps is that of cleaning a magnesium metal article with an aqueous acidic chemical cleaner followed by treating the said article with a dilute aqueous solution of CrO Among the advantages of the invention are the rapidity with which it may be carried out and the low cost of the overall treating and welding operation using the method of the invention.

Welding test results, Method of treating magnesium metal articles number at good spots made before- Test No.

Alkaline Aqueous acidic chemical Chromic acid treatment Tips struck Article Mechanical cleaning cleaning Solvent cleaning cleaning +CWR +CWR and HWR 3 eonsccuburned +CWR tive times CAN 2 min., 80 F"... 0.25% soln.. 30 sec. 7()80 F..- I 258+ o 0.5% soln., 30 sec. 7080 F. b 89 1% soln., 30 sec. Til- F b 76 0.25% soln., 30 sec, 1LT- Steel1 wire brushed l0 s. b 99 DS 1 Illll'L, R.T CAN 1 min., R.

50111., 30 see, R. 1, soln., 30 see, R. u 2

b 13 CS 3 min, 7080 F b 69 Test run in duplicate. b Average 012 tests. Average of 3 tests.

In each case electrode tips still in good condition, test stopped only because no more room on test piece for more welds.

4 Test continued on additional pieces until sticking cruised bad welds and electrode tips flattened.

No'rE.GWRCold water rinse. IIWR-Hot water rinse. GrO: 0.05% sulfuric acid. DS-Dilute sulfuric acid (0.5%).

CA N--Carboxyllc acid-nitrate soln. containing 20% acetic acid-5% NBNOQ. CS-l8% BT-Room temperature.

What is claimed is:

l. A method of treating a magnesium metal article prior to subjecting the article to electrical resistance spot welding, the said article having a clean and bright metal surface, comprising subjecting the said article to the action of an aqueous solution of chromic acid consisting essentially of from 0.5 to grams of dissolved CrO per liter of solution at a temperature in the range of 60 to 212 degrees Fahrenheit and for a period of from about 5 seconds to 3 minutes and rinsing the so-treated article with water.

2. A method of treating a magnesium metal article prior to subjecting the article to electrical resistance spotwelding, the said article having a clean and bright metal surface, comprising subjecting the said article to the action of an aqueous solution of chromic acid, consisting essentially of from 1.5 to 3 grams of dissolved CrO per liter of solution at a temperature in the range of 70 to 100 degrees Fahrenheit and for a period of from to 45 seconds and rinsing the so-treated article with water.

3. A method of treating the surface of a magnesium metal article prior to subjecting the said article to electrical resistance spotwelding comprising subjecting the article to the action of an alkaline cleaner at an elevated temperature for about 3 to 5 minutes, water rinsing the article, subjecting the article to the action of 0.5 percent References Cited in the file of this patent UNITED STATES PATENTS 1,795,473 Burdiclt Mar. 10, i931 2,302,939 De Long Nov. 24, 1942 2,346,562 De Long Apr. 11, 1944 2,430,435 Sperry Nov. 4, 1947 2,514,304 Bacon et a1. July 4, 1950 2,607,739 De Long Aug. 19, 1952 2,705,500 Deer Apr. 5, 1955 OTHER REFERENCES Survey on Welding Aluminum and Magnesium, Craighead, C. M., Office of Tech. Services, Wash., DC, 1947, pages 61-62 relied on.

Welding Handbook, 3rd ecL, American Welding Society, 1950, page 720 relied on. 

1. A METHOD OF TREATING A MAGNESIUM METAL ARTICLE PRIOR TO SUBJECTING THE ARTICLE TO ELECTRICAL RESISTANCE SPOT WELDING, THE SAID ARTICLE HAVING A CLEAN AND BRIGHT METAL SURFACE, COMPRISING SUBJECTING THE SAID ARTICLE TO THE ACTION OF AN AQUEOUS SOLUTION OF CHROMIC ACID CONSISTING ESSENTIALLY OF FROM 0.5 TO 10 GRAMS OF DISSOLVED CRO3 PER LITER OF SOLUTION AT A TEMPERATURE IN THE RANGE OF 60 TO 212 DEGREES FAHRENHEIT AND FOR A PERIOD OF FROM ABOUT 5 SECONDS TO 3 MINUTES AND RINSING THE SO-TREATED ARTICLE WITH WATER. 